Resistance welding processes are widely used in the manufacture of sheet metal assemblies, such as automobile and aircraft bodies. Each resistance weld involves a sequence of electrical energy and mechanical pressure steps. The sequence is provided by a controller which governs the timing of both the electrical and the mechanical steps. For example, the controller may establish a selected number of cycles of A.C. line power at a selected current, with a selected electrode force for each step of a welding operation. In carrying out the welding operation, the operator merely presses a button which sets the controller in operation. The controller then responds to an internal program to carry out all the steps necessary for the operation. The program of the controller is stored in an internal memory, and it may vary from a relatively few commands to a large number of commands, depending upon the simplicity or complexity of the welding operation.
The controller of the invention is controlled by a microprocessor, as mentioned above, which has programmable read-only and random access memories associated therewith. The controller provides heat and pressure commands to the controlled resistance welding machine. The controller includes a digital circuit for compensating for line voltage variations and disturbances, as mentioned above. This circuit digitizes the RMS input voltage and computes the proper firing time to maintain constant power during line voltage fluctuations and disturbances. The firing times are controlled by silicon controlled rectifiers or ignitrons, as will be described. Over-temperature protection of the devices may be provided in the manner described and claimed in U.S. Pat. No. 4,039,928, which is assigned to the present assignee.